Nearby three quarters of Sri Lanka lies in what is widely known as the 'Dry Zone', comprising the northern half and the whole of the east of the country. Average annual rainfall in this region is generally between 1,200-1,800 mm. In comparison with many parts of Europe this may not seem unduly dry, but like much of SE India, virtually all of the region's rain falls in the 3 months of the north-east Monsoon between October and December. The rain often comes in relativity short but dramatic bursts. Habarana, for example, located in the Dry Zone between Polonnarywa and Anuradhapura received 1,240 mm (nearly 50") of rain in the 3 days around Christmas in 1975. These rains caused catastrophic floods right across the Dry Zone.

The Wet Zone also receives some rain during this period, although the coastal regions of the South-west are in the rain shadow of the Central Highlands, and are much drier than the North-east between November and January. The South-west corner of Sri Lanka, the Wet Zone, has its main wet season from May to September, when the South-west Monsoon sweeps across the Arabian Sea like a massive wall of warm moist air, often over 10,000m thick. The higher slopes of the Central Highlands receive as much as 4,000 mm during this period, while even the coastal lowlands receive over 500mm.

From the October to December cyclonic storms often from over the Bay of Bengal, sometimes causing havoc from the southern coast of India northwards to Bangladesh. Sri Lanka is far enough south to miss many of the worst of these, but it occasionally suffers major cyclones. These generally come later in the season in December and January and can cause enormous damage and loss of life.